It is known, for example on a big project, to mix macadam, concrete, or the like on the spot. For concrete the various components--coarse and fine aggregate and cement--are carefully measured out, then blended with each other dry, then with water to form concrete. The advantage of mixing ones own concrete, for example, is that the concrete is fresher than ready mix and can be mixed differently for different applications. In addition in large quantities it is much cheaper to mix ones own concrete, as compared to buying ready-mix, which may not even be available.
This is accomplished by an apparatus which normally has three or four rigid prismatic hoppers or silos all carried on a frame and each provided at its base with a metering gate. This entire unit is delivered to the site, normally on a flat-bed semitrailer. Once installed, a mound of earth is built next to it so a front loader or the like can dump the various bulk materials into the respective hoppers. A weighing carriage or bucket car can move along the frame underneath the gates to receive from the hoppers, one at a time, predetermined volumes or masses of the respective bulk materials. Once filled with the appropriately proportioned mixture this carriage is dumped out into a blender or mixer.
Such an apparatus is quite difficult to set up and get into operation. Building the mound next to it so a filling device can ascend high enough to unload into the hoppers is a burdensome task, as the mound must be quite wide in order not to collapse under the weight of the loader. In addition the various hoppers are normally rather small, since they must be able to pass along the highways, that is they must be narrower than maximum truck width (about 8 ft) and similarly short. The standard apparatus also normally has a maximum of three bins or hoppers, so that if more are needed it is necessary to employ a wholly different piece of equipment.